Known in the art is a duplex stainless steel (Alloy 255) commercially identified under the trademark FERRALIUM.RTM. registered by Bonar Langley Alloys Ltd. The alloy is disclosed in U.S. Pat. No. 3,567,434 granted on Mar. 2, 1971. The background and development of the advantages of the duplex ferritic-austenitic structure is described in the patent.
Alloy 255 is used for welding operations as welding filler material. The all-weld-metal properties are generally acceptable except for toughness, ie, low impact strength. Thus, special attention must be made to weldments made with Alloy 255 as the weld filler material. This problem has been known in the art thereby tending to reduce the full marketing exploitation of welded duplex stainless steels.
Other duplex steels are described in U.S. Pats. Nos. 4,585,479 and 3,082,082 and U.K. Patent Application No. GB 2,160,221A.
U.S. Pat. No. 4,585,479 discloses an alloy system containing 7% to 12% nickel and specifically teaches against contents of molybdenum and copper.
U.S. Pat. No. 3,082,082 discloses an alloy system that did not appreciate the control of critcal nitrogen additions in a duplex steel.
U.K. Patent Application No. GB2,160,221A discloses a duplex stainless steel with a controlled limitation of aluminum content to improve impact properties.
Known in the art is UNS No. J-93345, a duplex stainless steel marketed as Escaloy D. The UNS composition of the alloy is, in weight percent, chromium 20-27, molybdenum 3 to 4.5, nickel 8 to 11, nitrogen 0.1 to 0.3, manganese 1.0 max., carbon 0.08 max., phosphorus 0.04 max., sulfur 0.025 max. and the balance iron. The typical Escaloy D alloy contains, in weight percent, 0.04 carbon, 24 chromium, 9.6 nickel, 3.4 molybdenum, 1.07 silicon, 0.55 manganese, 0.21 copper, 0.2 nitrogen, balance iron and containing about 235 PPM oxygen. The alloy may be a cast or wrought structural product, tubular or down hole equipment, for use in deep gas wells containing nominally about 30% hydrogen sulfide.
While these patents disclose compositions of duplex stainless steels, they do not solve the problems of welding said steels. These duplex steels have certain limitations, when welded, probably due, in part, to the nitrogen content. Generally in the welded condition: (1) the ductility and toughness are lower, (2) the postweld heat treatment results in reduced strength and (3) the corrosion resistance of the alloy is lowered. There is a need in the art for a welding filler material that has the properties of duplex stainless steel base materials, including corrosion resistance, ductility and high strength. Post welding tensile and impact strength are especially required.